WindowsXP reboots all the time

Hello,
I have a laptop that reboots all the time, I dont even see the welcome logo.
The only thing I see is the start window with the blue bar running from left to right... Then it restarts.
Here are somes explanations:
The laptop has been heavily virus infected, and since, I havent been able to open the system. I have used Dr. Web LiveCD, that has really wipped A LOT of dirt out of the drive (Many trojans, and among them PCDefender... ) But I still cant open XP. (Home edition by the way) What can I do?

Thanks for all the help!

 

A Repair install would seem to be in order.

If severely infected, it will require a full clean install.

 

Hmmm... I have managed to install another partition on the drive and install a clean XP. I was hoping I could repair somehow the faulty OS around this way... Do you have any suggestions?